Energy conservation, in all its varied forms, has become a national priority of the United States as well as the rest of the world, from both the practical point of view of limited natural resources and recently as a security issue to reduce our dependence on foreign oil. A large proportion (some estimates are as high as one third) of the electricity used in residential homes in the United States each year goes to lighting. The percentage is much higher for businesses, street lights, and other varied items. Accordingly, there is an ongoing need to provide lighting which is more energy efficient.
It is well known that incandescent light bulbs are very energy inefficient light sources - - - about ninety percent of the electricity they consume is released as heat rather than light. This heat adds to the cooling load of a system during cooling season. In heating season the cost per BTU of heat that the lights give off is typically more expensive than the cost per BTU of the main heat source. The heat that is given off by the lighting also can cause “over shooting” of the desired temperature which wastes energy and makes the space feel uncomfortable. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about four) but are still quite inefficient as compared to solid state light emitters, such as light emitting diodes (LEDs).
In addition, as compared to the normal lifetimes of solid state light emitters, incandescent light bulbs have relatively short lifetimes, i.e., typically in the range of 750 to 2000 hours. Fluorescent bulbs have longer lifetimes (e.g., 8,000 to 20,000 hours), but provide less favorable color reproduction and contain hazardous mercury. In dramatic comparison, the lifetime of light emitting diodes, for example, can generally be measured in decades (approximately 50,000 hrs or more).
One established method of comparing the output of different light generating sources has been coined color reproduction. Color reproduction is typically given numerical values using the so-called Color Rendering Index (CRI). CRI is a relative measurement of how the color rendition of an illumination system compares to that of a blackbody radiator, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI equals 100 if a set of test colors being illuminated by an illumination system are the same as the results as being irradiated by a blackbody radiator. Daylight has the highest CRI (100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70 to 85). Certain types of specialized lighting devices have relatively low CRIs (e.g., mercury vapor or sodium, both as low as about 40 or even lower). Sodium lights are used, for example, to light highways and surface streets. Driver response time, however, significantly decreases with lower CRI values (for any given brightness, legibility decreases with lower CRI).
A practical issue faced by conventional lighting systems is the need to periodically replace the lighting devices (e.g., light bulbs, fixtures, ballasts, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on a typical usage of 6 hours per day for 20 years). In contrast, light-producing device lifetimes are typically much shorter, thus creating the need for periodic change-outs. The potential number of residential homes that may be candidates for these periodic change-outs of the traditional incandescent lighting systems, including base fixtures and lamps themselves, may be extremely large and represent an attractive commercial enterprise. For example, in the United States alone new residential home construction has an average of approximately 1.5 million dwellings per year over the last 30 years. Including older homes built before 1979, this represents at least 100 million residential dwellings that are candidates for potential upgrades to more energy efficient LED-based lighting systems.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where solid state light emitters are already being used, efforts are ongoing to provide solid state light emitter-containing devices which have improved energy efficiency, color rendering index (CRI), contrast, and useful lifetime.
Light emitting diodes are well-known semiconductor devices that convert electrical current into light. A wide variety of light emitting diodes are used in increasingly diverse fields for an ever-expanding range of purposes. More specifically, light emitting diodes are semiconducting devices that emit light (ultraviolet, visible, or infrared) when an electrical potential difference is applied across a p-n junction structure. There are a number of well-known ways to make light emitting diodes and many associated structures, and the present invention can employ any such manufacturing technique.
The commonly recognized and commercially available light emitting diodes that are sold, for example, in electronics stores typically represents a “packaged” device made up of a number of parts. These packaged devices typically include a semiconductor-based light emitting diode and a means to encapsulate the light emitting diode. As is well known, a light emitting diode produces light by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer. The electron transition generates light at a wavelength that depends on the band-gap energy difference. Thus, the color of the light (usually expressed in terms of its wavelength) emitted by a light emitting diode depends on the semiconductor materials embedded in the active layers of the light emitting diode.
Although the development of solid state light emitters, e.g., light emitting diodes, has in many ways revolutionized the lighting industry, some of the characteristics of solid state light emitters have presented challenges, some of which have not yet been fully met. For example, the emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, e.g., for providing lighting, given that such an emission spectrum typically provides a very low CRI.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode can produce white light. “White light” emitting devices have been produced which have a light emitting diode structure comprising individual red, green and blue light emitting diodes mounted on a common substrate. Other “white light” emitting devices have been produced which include a light emitting diode which generates blue light and a luminescent material (typically, a phosphor) that emits yellow light in response to excitation by the blue LED output, whereby the blue and the yellow light, when appropriately mixed, produce light that is perceived by the human eye as white light.
A wide variety of luminescent materials are well-known and available to persons of skill in the art. For example, a phosphor is a luminescent material that emits a responsive radiation (typically visible light) when excited by a source of exciting radiation. In most instances, the responsive radiation has a wavelength, which is typically longer, than the wavelength of the exciting radiation. Other examples of luminescent materials include day glow tapes and inks, which glow in the visible spectrum upon illumination by ultraviolet light. Luminescent materials can be categorized as being down-converting, i.e., a material which converts photons to a lower energy level (longer wavelength) or up-converting, i.e., a material which converts photons to a higher energy level (shorter wavelength). Inclusion of luminescent materials in LED devices has typically been accomplished by adding the luminescent materials to a clear plastic encapsulating material (e.g., epoxy-based or silicone-based material).
As noted above, “white LED lights” (i.e., lights which are perceived as being white or near-white by the human eye) have been investigated as potential replacements for white light incandescent lamps. A representative example of a white LED light includes a package of a blue light emitting diode chip, made of gallium nitride (GaN), coated with a phosphor such as Yttrium Aluminum Garnet (YAG). In such an LED light, the blue light emitting diode chip produces a blue emission and the phosphor produces a yellow fluorescence on absorbing that blue emission. For instance, in some designs, white light emitting diodes are fabricated by forming a ceramic phosphor layer on the output surface of a blue light-emitting semiconductor light emitting diode. Part of the blue rays emitted from the light emitting diode pass through the phosphor, while another part of the blue rays emitted from the light emitting diode chip are absorbed by the phosphor, which becomes excited and emits a yellow ray. The part of the blue light emitted by the light emitting diode, which is transmitted through the phosphor, is mixed with the yellow light generated by the phosphor. The human eye perceives the mixture of blue and yellow light as white light.
In another type of LED lamp, a light emitting diode chip that emits an ultraviolet ray which is absorbed by a phosphor material that produces red (R), green (G) and blue (B) light rays. In such an “RGB LED lamp”, the ultraviolet rays that have been radiated from the light emitting diode excites the phosphor, causing the phosphor to emit red, green and blue light rays which, when mixed, are perceived by the human eye as white light. Consequently, white light can also be obtained as a mixture of these light rays.
Designs have been realized in which existing LEDs and other electronics are assembled into an integrated housing fixture. In such designs, an LED or plurality of LEDs are mounted on a circuit board encapsulated within the housing fixture, and a heat sink is typically mounted to the exterior surface of housing fixture to dissipate heat generated from within the device, the heat being generated by inefficient AC-to-DC conversion from within the device. Although devices of this type can generate white light by any of the means described above, their external geometry typically does not permit direct functional replacement of existing incandescent lighting systems currently installed in residential homes. For example, one such prior art device is described in the CREE Lighting Fixtures Inc. catalog as part number LR6. The LR6 embodiment includes an encapsulated LED structure with an external heat sink assembly integrated as part of a thermal management system. The necessity of an external heat sink assembly in conjunction with an integrated thermal management system adds significant cost to the device as compared to equivalent light output off-the-shelf incandescent devices. In addition, the incorporation of the external heat sink assembly adds significant weight to the device as well as yields an overall external geometry to the lamp which is cylindrical in nature, not at all similar to the familiar incandescent lamps. This unusual aesthetic appearance may be an impediment to market acceptance by the average home owner envisioning a direct swap-out.
In addition to the above drawbacks, currently available LED-based lighting devices do not appear to generate sufficient light output, at a cost competitive price, to be a direct lumen-for-lumen replacement for incandescent lighting devices. This may be one of the biggest reasons for current poor market penetration of white-light LED lighting devices into the residential marketplace.
Another drawback with conventional LED lamps is the undesirable creation of shadows and hot spots. For example, the light generated by the individual LED elements can be clearly seen by the human eye as a bright (hot) spot. These bright spots create corresponding bright/hot spots on surrounding surfaces being illuminated and the areas between these bright spots appear to be shadowed. This is in contrast to the relatively even dispersion of light generated by incandescent bulbs.
Yet a further drawback of conventional LED lamps is the need to design and manufacture a unique lamp system for each different size/shape and/or wattage of bulb. Some of the more common bulb shapes are American National Standards Institute (ANSI) PAR30, PAR 38, R20 and MR16. Thus, conventional LED lamps for each of these shapes typically have a proprietary light engine and housing. This results in additional engineering, parts and manufacturing costs.
Given the above-noted concerns, there is a need for an improved LED-based white light illumination device that overcomes, at least in part, the disadvantages of the prior art lighting systems, including the prior art LED-based lighting systems.